[NTG-context] figure numbering S3, S4, S6, ...

Dear list, I am compiling some figures as supplementary material and would like to have figure captions follow the convention: Figure S3 Figure S4 Figure S5 ... and I need that the numbering starts with S3 instead of S1. How can I achieve this? Minimal example: \setuppapersize[A6] \setupexternalfigures[location=default] \setupcaptions[way=bytext] \placefigure {Cow} {\externalfigure[cow][width=2cm]} \placefigure {Another Cow} {\externalfigure[cow][width=2cm]}